Electrical arcs can develop temperatures well above the ignition level of most common flammable materials and, therefore, pose a significant fire hazard. For example, worn power cords in the home may arc sufficiently to start a fire. Fortunately, low-voltage arcing is an inherently unstable phenomenon and does not usually persist long enough to start a fire. Under certain conditions, reflected in particular characteristics of the electrical disturbance produced, the likelihood of the arc persisting and starting a fire is much higher.
Two types of dangerous arcing that are likely to occur in the home are momentary, high-energy arcs caused by high-current faults and persistent, low-current “contact” arcing. A high-current fault, caused by an inadvertent direct connection between line and neutral or line and ground, will generally draw current up to or beyond the rated capacity of the circuit, arc explosively as the contacts are physically made and broken, dim lights and other loads indicating an excessive load is being drawn, and/or (assuming the circuit is properly protected by a circuit breaker) trip the breaker, thereby interrupting the current to the arc. Because such “line faults” are short-lived, the temperature rise in the feed conductors is limited and the fire danger results primarily from the explosive expulsion of minute glowing globules of copper from the contact area which may ignite nearby flammable materials. Even if a fire begins, however, the high visibility of the fault and the likely presence of someone nearby (who provided the physical impetus to bring the conductors together) sharply mitigates the potential for an uncontrolled fire.
Contact arcing, on the other hand, is arcing that occurs at connections in series with a load. As such, the maximum current in the arc is limited to the load current and, therefore, may be substantially below the overcurrent or “trip” rating of an associated circuit breaker. Contact arcing is a complex physical phenomenon that may be induced by loose connections, oxidized contacts, foreign non-conducting material interfering with the conduction path, differences in contact materials, contact shapes, and other factors. Under certain conditions, such arcing may become persistent and present a substantial fire hazard.
One example of a condition that may cause contact arcing is a well-used wall outlet wherein the spring pressure provided by the contacts has been reduced through age and use, so that insufficient pressure is applied to the inserted plug contacts to ensure low-resistance connection.
Contact arcing is also commonly caused by use of extension cords of insufficient current-carrying capacity. For example, the plug may be heated by resistance heating, gradually decomposing elastomeric insulating material around the contacts until the material partially flows into the contact area, preventing proper contact from being made. This process may become regenerative as the initial arcing produces more heat, carbonizing the insulation and producing a thin insulating layer on the contact surface.
A third cause of contact arcing often observed in aluminum wiring involves the oxidation of contacts. In this case a chemical process, principally oxidation, builds up a semi-conductive or non-conductive layer on the surface of the contacts. Preferably, when the contact material is susceptible to oxidation, the connection is made gas-tight to prevent oxygen from entering and promoting oxidation. However, if the connections become loose over time, oxidation begins and arcing can result.
Contact arcing is also common when the springs that snap switches on or off become worn, increasing the time to closure and reducing the force that holds the contacts together.
A fifth example of contact arcing that is found to readily occur in residences is at the center contact of conventional light bulbs. Because the center contact is subjected to high temperatures and repeated use, it often becomes loose and oxidizes, thereby increasing the likelihood of arcing. When arcing occurs, the lamp contact, usually made of a low-melting-point solder, melts and reforms, either breaking the contact or establishing a new one. A common result in very old lamp fixtures is that arcing at the center contact or around the aluminum threads causes the lamp to become welded into the socket and therefore very difficult to remove.
Finally, high-resistance faults across the line are, in the present context, also considered contact arcing. Inadvertent “shorts” that exhibit enough resistance to prevent tripping of the circuit breaker may nonetheless produce arcing at the contact points, and are considered contact arcs. Frayed conductors that come into light or intermittent contact, or staples that inadvertently pierce wire insulation, can produce resistive shorts through contamination and oxide layers, particularly if moisture is present.
Most instances of contact arcing result from the gradual degeneration of current-carrying contacts. Dangerous arcs may begin as small and occasional arcing, gradually building up over time until the arcing becomes persistent enough to start a fire. Also, in sharp contrast to the visible nature of arcing produced by line faults, such as “hard” or “bolted” shorts, incipient contact arcing is often hidden from view, providing little or no indication of the impending danger. For this reason, it would be highly advantageous if contact arcing conditions could be detected early, and a warning provided before the danger due to the fault reaches a dangerous level.
It will thus be appreciated that there are fundamental differences between “hard shorts” and contact arcing. “Hard shorts” will generally involve high currents (>50 A) and will be explosive at the fault point contact, so that the fault will either burn itself out or trip a circuit breaker. Conventional circuit protection devices are normally adequate to guard against line fault arcing. By comparison, the average current drawn in contact arcing is no more than the current drawn by the load itself. Nevertheless, even low-current contact arcing, for example, a 60 watt light bulb on the end of a faulty extension cord, or a set of Christmas tree lights with faulty contacts, may release sufficient heat to cause a fire. Accordingly, conventional circuit breakers are inadequate to prevent dangerous conditions due to contact arcing.
A need also exists for a circuit breaker that, in addition to detecting arcing that may result in a fire, removes power from the load when hazardous arcing is present. Such a device could be conveniently packaged in much the same style as a conventional circuit breaker, or could be installed in an outlet similar to the currently available ground fault interrupters. Because the load current flows through the circuit breaker, it is convenient in this application to monitor load current.
A need also exists for an arc detector that is immune to noise commonly present on household power lines, e.g., due to lamp dimmers, brush motors, carrier-current communications systems, switching transients, broadcast radio signals, and other types of noise signals that may have similar electrical characteristics as arc-faults. If not properly identified and rejected, these types of signals, which may be easily confused with arc-fault signals, may cause “nuisance” tripping of certain arc-fault circuit detectors. Accordingly, in an effort to reduce the negative effects of nuisance tripping and accurately respond to arc fault signals, systems and methods for identifying arc-faults in power systems may be required.
Current AFCI/GFCI breakers may only display the last trip condition after a fault event has occurred through the use visual indicators (i.e. flags). The indication is retained until the device is reset and turned back on. After the indication is cleared, however, there is no record of the event until another occurrence has been detected.
It may be advantageous to incorporate arc-fault detection and ground fault detection capabilities into a single, integrated module, thereby reducing consumer costs associated with installing, maintaining, and servicing multiple circuit interrupting devices on a single branch. Furthermore, by combining arc-fault and ground-fault detection functions within a single module, many of the processing functions associated with arc-fault detection such as, for example, electronic fault monitoring, self-test functionality, and fault event data recording, may also be implemented in ground fault detection processes to enhance existing ground fault detection capabilities.